Using unity tree editor with blender trees
Every object in Blender has a center point or origin for it's local coordinates in the 3D view

It is generally represented by a large orange dot by default. You have to have this into consideration when modeling.

Your tree's origin point is likely placed a the center or the tree instead of the trunk base.

Change this either by moving it's gemetry in edit mode so the center is at the bottom of the trunk, or place the 3D cursor where you want the center to be (for example select a face press Shift + S Cursor to Selection) and then exit Edit Mode and press Shift + Ctrl + Alt + C and choose Origin to 3D Cursor